Our law enforcement officers undergo so much training to stop a killer or killers on the loose that they have acquired considerable skill at it.

For instance, the first District of Columbia Police officers to arrive at the facility — including mountain bike officer Michael Wear, special operations officer Vernon Dallas and canine officer Scott Williams — didn't wait for reinforcements to enter Building 197, according to city officials.

When they heard gunshots, they assumed people were in danger. So they went in, just as D.C. Police officers are taught. A short time later, Williams was shot in the legs.

The decision to enter right away might seem obvious, but in fact it's been the norm only since the 1999 high school massacre in Columbine, Colo. That's when U.S. police departments realized they were leaving killers too much time by sticking to the previous practice of waiting for SWAT teams or other specialized units to arrive.

"That is the training. This is a homicide in progress. We don't wait," D.C. Police Chief Cathy Lanier said.

Some glitches occurred Monday. It's too early to give a final grade to District of Columbia Police and at least eight other federal, state and local forces that rushed to the scene.

"From everything I've seen, their response time was pretty incredible. They didn't wait to set up a perimeter or stage officers. They actively went in and pursued the bad guy. That, in and of itself, saves lives," said Richard Beary, police chief at the University of Central Florida and second vice president of the International Association of Chiefs of Police.

"I think the training that the FBI has done, the federal law enforcement centers have done, has clearly made it down to the rank and file," he said.

Don Alwes, the lead instructor on "active shooter" incidents for the National Tactical Officers Association, said: "Have we learned anything? You bet we have. You can't imagine how many people this guy would have killed had he gotten in there 10, 15 years ago. Prior to Columbine, this would have been a totally different response."

Alwes and others also praised what appeared to be smooth cooperation among multiple agencies. The group that ultimately brought down the psychologically troubled shooter, Aaron Alexis, included officers from both D.C. Police and U.S. Park Police.

"These young men and women quickly paired up and ran toward danger, ran toward the sound of shots. They worked together regardless of the patch on the sleeve or the shield on the door," Park Police Chief Teresa Chambers said.

There were problems, of course. Union leaders said emergency radios used by some Navy police officers and firefighters didn't work properly.

There was confusion for most of the day Monday over whether as many as two shooters, in addition to Alexis, might have been involved. But such false concerns often arise in the turmoil surrounding violent events, and it was better to err on the side of caution.

Finally, an investigation is underway into reports that U.S. Capitol Police commanders might have erred by withdrawing a tactical team from the Navy Yard, where it could have provided early assistance.

However, some experts said it might have been wise at the time to keep the team close to the Capitol in case the Navy Yard attack was just the start of a series of coordinated terrorist assaults.

The after-action inquiry led by the FBI must address these concerns and any new ones that arise.

For now, Lanier and others were emphasizing the scale of the achievement. The chief was especially impressed because the 12 small "active shooter teams" that hunted Alexis were operating in such a challenging setting.

"That building is literally a maze of 1 million square feet of cubicles," she said. "It is absolutely the worst environment you can imagine in combatting an active shooter, especially one who had some familiarity with the building."

She said training and preparation have improved steadily over time, as new lessons are learned with each incident.

"It changes after every shooting," she said. "Every officer I've talked to said the same thing: The training was key."